U.S. patent number 6,646,281 [Application Number 10/094,305] was granted by the patent office on 2003-11-11 for differential detector coupled with defocus for improved phase defect sensitivity.
This patent grant is currently assigned to KLA-Tencor Corporation. Invention is credited to Matthias C. Krantz, Damon F. Kvamme, Donald W. Pettibone, Stan Stokowski.
United States Patent |
6,646,281 |
Krantz , et al. |
November 11, 2003 |
Differential detector coupled with defocus for improved phase
defect sensitivity
Abstract
Provided are apparatus and methods for detecting phase defects.
The invention relies generally on the distortion of light as it
passes through defects in phase shift masks to detect these
defects. Light traveling through a defect, such as a bump in an
etched area will travel at a different rate than light traveling
through air. In order to enhance the signals generated from the
defects, the invention in several embodiments provides a defocused
light inspection beam by setting the focus of the beam to a level
above or below the photomask subject to inspection. The light from
the photomask is collected by a detector split into at least two
portions, each generating a signal. A resulting differential signal
produced from the signals generated at each of the two detector
portions is used to determine whether a defect in the photomask is
present, in one embodiment, by generating an image from the
resulting signal.
Inventors: |
Krantz; Matthias C. (Altenholz,
DE), Pettibone; Donald W. (San Jose, CA), Kvamme;
Damon F. (San Jose, CA), Stokowski; Stan (Danville,
CA) |
Assignee: |
KLA-Tencor Corporation (San
Jose, CA)
|
Family
ID: |
26788719 |
Appl.
No.: |
10/094,305 |
Filed: |
March 7, 2002 |
Current U.S.
Class: |
250/559.45;
348/125 |
Current CPC
Class: |
G01N
21/95607 (20130101); G03F 1/84 (20130101); G01N
2021/95676 (20130101) |
Current International
Class: |
G01N
21/88 (20060101); G01N 21/896 (20060101); G01N
21/956 (20060101); G01B 009/02 () |
Field of
Search: |
;250/548,559.45,559.46
;348/125,126,128 ;356/399-401,237.1-237.5,394 ;382/144 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Beyer Weaver & Thomas, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application takes priority under U.S.C. 119(e) of U.S.
Provisional Application No.: 60/344,225 filed Dec. 28, 2001
entitled, "DIFFERENTIAL DETECTOR COUPLED WITH DEFOCUS FOR IMPROVED
PHASE DEFECT SENSITIVITY" by Matthias C. Krantz, Donald W.
Pettibone, Damon F. Kvamme and Stan Stokowski, which is
incorporated by reference in its entirety for all purposes.
Claims
What is claimed is:
1. A method for detecting phase defects in a semiconductor
processing photomask, the method comprising: setting a focus of an
optical inspection beam to a predetermined distance above or below
a surface of semiconductor processing photomask, collecting light
reflected from or transmitted through the semiconductor processing
photomask at a detector comprising at least two portions,
generating a first signal from a first portion of the at least two
portions and a second signal from a second portion of the at least
two portions, and obtaining the difference between the first signal
and the second signal to produce a resulting signal indicating
whether there is a defect present.
2. The method recited in claim 1 wherein the predetermined distance
above or below the surface of the semiconductor processing
photomask is selected so that the optical inspection beam is
defocused.
3. The method recited in claim 1 wherein the first portion
comprises a circular region and the second portion comprises an
annular region outside the first portion and wherein both portions
are concentric.
4. The method recited in claim 3 wherein the first and second
portions are separated by a third portion which is annular in shape
and concentric with the first and second portions.
5. The method recited in claim 3 wherein the diameter of the first
portion is approximately 0.7 times the outer diameter of the second
portion.
6. The method recited in claim 3 wherein the diameter of the first
portion is approximately 0.3 times the outer diameter of the second
portion and the inner diameter of the second portion is
approximately 0.8 times the outer diameter of the second
portion.
7. The method recited in claim 3 wherein the detector further
comprises a third portion and a fourth portion and further
comprising generating the first signal from the first portion and
the third portion and the second signal from the second portion and
the fourth portion.
8. The method recited in claim 7 wherein each of the third and
fourth portions are annular regions and wherein the third portion
is positioned between the first and second portions and the fourth
portion is positioned outside the second portion.
9. The method recited in claim 7 wherein the first portion and the
third portion are not contiguous.
10. The method recited in claim 7 wherein the first portion and the
third portion are separated by an annular isolation region.
11. The method recited in claim 7 wherein at least two of the
first, second, third, and fourth portions are separated by annular
shaped isolation regions.
12. The method recited in claim 7 wherein the first portion and the
third portion are contiguous.
13. The method recited in claim 1 wherein the first portion and the
second portion are approximately equal in area.
14. The method recited in claim 1 wherein the predetermined
distance lies in the range of 200 to 500 nm.
15. The method recited in claim 1 wherein the predetermined
distance lies in the range of 0.5 to 3 times a wavelength of the
optical inspection beam divided by the square of the numerical
aperture of an objective lens used to diffract the optical
inspection beam directed towards the photomask.
16. The method recited in claim 1 wherein the setting a focus and
collecting light reflected from or transmitted through a
semiconductor processing photomask is performed using a combination
of objective lens and collection lens producing a sigma value from
about 0.2 to about 0.7.
17. The method recited in claim 1 wherein the resulting signal is
used to generate an image.
18. The method for detecting phase defects in a semiconductor
processing photomask in a first mode recited in claim 1, the method
further comprising in a second mode: setting a focus of an optical
inspection beam to a surface of the semiconductor processing
photomask, collecting light reflected from or transmitted through
the semiconductor processing photomask at a detector comprising a
first portion and a second portion, wherein each of the first and
second portions generates a signal, and combining by addition the
signal from the first portion and the signal from the second
portion of the detector to produce an image of the semiconductor
processing photomask.
19. A method for detecting phase defects in a semiconductor
processing photomask, the method comprising: setting a focus of an
optical inspection beam of an inspection system having an objective
lens and a collection lens to a predetermined distance above or
below a surface of semiconductor processing photomask, collecting
light reflected from or transmitted through the semiconductor
processing photomask at a detector comprising a plurality of
portions, wherein the collection lens, objective lens, and detector
configuration are related by a sigma value, generating a first
signal from at least a first portion, alone or in combination with
one or more of the plurality of portions, and a second signal from
at least a second portion of the plurality of portions, one of the
plurality of portions being circular in shape and the remaining
portions of the plurality having annular shapes and obtaining the
difference between the first signal and the second signal to
produce a resulting signal indicating whether there is a defect
present.
20. The method recited in claim 19 wherein the first signal and the
second signal are generated from portions of the plurality of
detector portions selected to correspond to a predetermined sigma
value.
21. The method recited in claim 19 wherein the portions of the
plurality of detector portions selected for generation of the first
and second signals are selected to compensate for changes in the
objective lens used in the system.
22. The method recited in claim 21 wherein portions of the
plurality of detector portions are selected near the center of the
detector to compensate for a reduced NA of the objective lens.
23. A method for detecting phase defects in a semiconductor
processing photomask, the method comprising: producing an image
using a Zernike phase shift plate on an inspection light beam
directed to a semiconductor processing photomask, collecting light
reflected from or transmitted through the semiconductor processing
photomask at a detector comprising a first portion and a second
portion, wherein each of the first and second portions generates a
signal, and obtaining the difference between the signal from the
first portion and the signal from the second portion of the
detector to produce a resulting signal indicating whether there is
a defect present.
24. The method recited in claim 23 wherein the first portion
comprises a circular region and the second portion comprises an
annular region outside the first portion and wherein both portions
are concentric.
25. The method recited in claim 24 wherein the first and second
portions are separated by a third portion which is annular in shape
and concentric with the first and second portions.
26. The method recited in claim 23 wherein the first portion and
the second portion are approximately equal in area.
27. The method recited in claim 24 wherein the diameter of the
first portion is approximately 0.7 times the outer diameter of the
second portion.
28. The method recited in claim 25 wherein the diameter of the
first portion is approximately 0.3 times the outer diameter of the
second portion and the inner diameter of the second portion is
approximately 0.8 times the outer diameter of the second
portion.
29. The method recited in claim 23 wherein the resulting signal is
used to generate an image.
30. A scanning optical microscopy system configured to detect phase
defects in a semiconductor processing photomask comprising: an
optical beam generator to direct a light beam towards a surface of
the semiconductor processing mask; a detector comprising a first
portion and a second portion, each of the first portion and the
second portion generating a signal from light collected from a
semiconductor processing photomask; a controller for setting a
focus of the light beam to a predetermined distance above or below
the surface of the semiconductor processing photomask; and an
analyzer configured to obtain the differences in the signals from
the first portion and the second portion of the detector.
31. The scanning optical microscopy system recited in claim 30
wherein the first portion comprises a circular region and the
second portion comprises an annular region outside the first
portion and wherein both portions are concentric.
32. The scanning optical microscopy system recited in claim 30
wherein the first and second portions are separated by a third
portion which is annular in shape and concentric with the first and
second portions.
33. The scanning optical microscopy system recited in claim 30
wherein the first portion and the second portion are approximately
equal in area.
34. The scanning optical microscopy system recited in claim 31
wherein the diameter of the first portion is approximately 0.7
times the outer diameter of the second portion.
35. The scanning optical microscopy system recited in claim 31
wherein the diameter of the first portion is approximately 0.3
times the outer diameter of the second portion and the inner
diameter of the second portion is approximately 0.8 times the outer
diameter of the second portion.
36. The scanning optical microscopy system recited in claim 31
further comprising an image generator for using the resulting
signal to generate an image.
37. The scanning optical microscopy system recited in claim 30
wherein the controller is further configured for setting the focus
of the light beam to a surface of the semiconductor processing
photomask and wherein the analyzer is further configured to
selectively obtain the sum of the signals from the first portion
and the second portion of the detector.
38. A scanning optical microscopy system configured to detect phase
defects in a semiconductor processing photomask comprising: an
optical beam generator to direct a light beam towards a surface of
the semiconductor processing mask; a detector comprising a first
portion and a second portion, each of the first portion and the
second portion generating a signal from light collected from a
semiconductor processing photomask; a complex amplitude plate
located on the incident plane to generate a defocused image; and an
analyzer configured to obtain the differences in the signals from
the first portion and the second portion of the detector.
39. The system recited in claim 38 wherein the complex amplitude
plate is a Zernike plate.
Description
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for
detecting defects in masks used in semiconductor processing. More
particularly, the present invention relates to apparatus and
methods for detecting defects in phase shift masks.
Fabrication of semiconductor wafers typically relies on
photolithography to produce circuit patterns on layers of a wafer.
The wafer is coated with a "photoresist". Light is then transmitted
through the mask and imaged onto the wafer. Photoresist is a
material which is sensitive to light. A negative photoresist cures
or hardens when exposed to light, so that the unexposed areas can
be washed away. For example, in one system, ultraviolet light is
used to expose a portion of the photoresist layer. A positive
photoresist reacts in the opposite manner, the exposed regions can
be washed away. The photoresist that is left acts as a mask, so
that materials may be deposited in the areas not covered by
photoresist to thereby form patterns on the wafer. The photoresist
is then removed.
Designers and manufacturers constantly strive to develop smaller
devices from the wafers, recognizing that circuits with smaller
features generally have greater speeds and increased yield (numbers
of usable chips produced from a standard semiconductor wafer).
Photolithography equipment manufacturers have generally employed
equipment using progressively smaller wavelengths to a current size
below 193 nm in order to achieve smaller feature sizes. However, as
the size of the circuit features decrease, physical limits such as
the convergence between the wavelength of the light used to create
the photoresist mask and the wafer feature sizes present obstacles
to further reduction in feature size using the same semiconductor
fabrication equipment.
Designers of such equipment have discovered that a phase shift mask
(PSM) will allow the patterning of smaller features, even as the
feature size approaches the wavelengths of the light used to create
the photoresist pattern from the PSM. In some cases the use of a
PSM may decrease the minimum feature size by a factor of two. With
PSM, the mask no longer looks like the design shapes. Instead, the
PSM contains shapes which cause the design shapes to appear as a
result of constructive and destructive interference of light
passing through the PSM. Alternating phase shift masks generally
use an etching technique to etch a small depression into the mask.
Light passing through the depression experiences a phase shift
relative to the unetched areas, creating sharper images at the
wafer. While the overall process uses particular design rules for
minimum feature size, a PSM allows circuits with more aggressive
critical dimensions to be consistently built using existing
lithography tools.
Due to their importance in decreasing feature size while using
existing equipment, semiconductor and semiconductor equipment
manufacturers are highly motivated to detect PSM defects. Given the
small size of the features and the volumes of wafers to be produced
from a mask, it is essential that defects be detected in the masks
to either enable repairs, where appropriate, of the mask or discard
unsalvageable masks prior to production.
Conventional inspection techniques such as optical methods work
well in identifying defects of typical chrome on glass masks. These
defects include the placement of chrome in unintended places and
the absence of chrome portions where desired. A conventional chrome
on glass mask is shown in FIGS. 1A and 1B. FIG. 1A shows a cross
section with chrome sections 102 and 104 deposited on transparent
layer 106. A typical material for layer 106 is quartz, due to its
ability to transmit light. FIG. 1B is a top view of the photomask
showing a typical defect 108. Conventional optical inspection
techniques work well in identifying such defects because the
amplitude of the light transmitted through the defect is directly
affected, i.e., the absence of the normally opaque chrome section
allows light to be transmitted and detected in a location where
such detection is unexpected. Contaminants on the glass can be
identified by using either transmitted or reflected light or a
combination of the two. The defects directly affect the amplitude
of light passing through and reflected from the mask and are
amenable to measurement by the above referenced conventional
techniques.
Phase shift defects, however, present unusual problems. Imaging of
phase objects and detection of phase defects typically requires
special imaging methods to convert the phase information into
intensity differences at the imaging detector. Numerous methods
have been proposed to accomplish this including the Zernike phase
contrast, differential interference contrast (DIC), differential
phase contrast (DPC), defocused imaging, and interferometric
techniques. Most of these methods involve changing the phase delay
of the optical wavefront in the pupil plane of the imaging system
in a way that will produce the greatest intensity effect at the
detector for a given phase defect or phase object. The optimum
method greatly depends on the phase shifts present in the object.
In biological samples weak phase shifts need to be imaged. In phase
shift masks for photolithography strong phase shifters are used. As
a result, a sensitive defect detection system for phase defects on
phase shift masks must detect weak phase objects in the presence of
strong phase and amplitude objects. Of particular interest in the
design is the response of a system to phase edges. Another
important aspect for automated photomask inspection systems is
whether the system response to phase objects is isotropic in the
plane of the object. An anisotropic response as yielded by the DIC
or a Nomarski technique or the linear DPC technique may be
acceptable for visual inspection but complicates automated
inspection.
For the foregoing reasons, there is a need for improved methods and
apparatus capable of detecting phase shift mask defects in the
presence of both weak and strong phase shifts.
SUMMARY OF THE INVENTION
To achieve the foregoing, the present invention provides apparatus
and methods for detecting phase defects.
The invention relies generally on scanner type imaging systems and
detects phase defects on photomasks by their modification of the
light passing through the mask. Much of the detail provided in the
specification for practicing the invention is given for scanners.
Those skilled in the art, having the benefit of the details
provided in this specification, will appreciate that the invention
may also be implemented on projector type imaging systems.
Specifically, the invention relies on the modification of the phase
of the wavefront at the pupil plane of the optical imaging system
using defocus in conjunction with the differential detection of the
image intensity from different segments of a detector.
In order to enhance the signals generated from phase defects, the
invention in several embodiments provides a defocused light
inspection beam by setting the focus of the scanning beam to a
level above or below the photomask subject to inspection, resulting
in a defocused beam. The light from the scanning beam reflected or
transmitted by the photomask is collected by a detector split into
at least two portions, each generating a signal. The detector is
typically positioned at or near the pupil plane. A resulting
differential signal produced from the signals generated at each of
the two detector portions is used to determine whether a defect in
the photomask is present, in one embodiment, by generating an image
from the resulting signal.
In one aspect, the first portion of the detector is a circular
region and the second portion is an annular region outside the
first region.
In one aspect the invention provides a method for detecting phase
defects in a semiconductor processing photomask by using a complex
amplitude plate such as a Zernike phase shift plate or similar
phase plate. The beam generated using a Zernike phase plate or
similar phase plate is used to derive a differential signal from
the two portions of the detector to produce the resulting signal
indicating whether phase defects are present.
In another aspect the invention provides a method of detecting
phase defects in a first mode using a defocused light beam and a
differential signal from the two portions of the detector. In a
second mode, detecting pattern defects occurs by adjusting the
focus of the optical inspection beam to the surface of the
photomask and by summing the signals from the first and second
portions of the detector to generate the resulting signal to
produce at-focus, normal imaging for Cr defects.
These and other features and advantages of the present invention
are described below with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural elements,
and in which:
FIGS. 1A and 1B illustrate a conventional chrome on glass
photomask.
FIGS. 2A and 2B depict a cross section of a photomask with etched
phase shift sections.
FIG. 3 is a diagram of a scanning optical microscopy system in
accordance with one embodiment of the present invention.
FIGS. 4A, 4B and 4C depict top views of a split detector in
accordance with embodiments of the present invention.
FIG. 5 is a diagrammatic representation of a conventional optical
microscopy system utilizing a Zernike plate.
FIG. 6 is a flowchart illustrating a procedure for inspecting phase
shift defects and pattern defects in accordance with one embodiment
of the present invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
Reference will now be made in detail to specific embodiments of the
invention. Examples of these embodiments are illustrated in the
accompanying drawings. While the invention will be described in
conjunction with these specific embodiments, it will be understood
that it is not intended to limit the invention to these
embodiments. On the contrary, it is intended to cover alternatives,
modifications, and equivalents as may be included within the spirit
and scope of the invention as defined by the appended claims. In
the following description, numerous specific details are set forth
in order to provide a thorough understanding of the present
invention. The present invention may be practiced without some or
all of these specific details. In other instances, well known
process operations have not been described in detail in order not
to unnecessarily obscure the present invention.
Several embodiments of the present invention rely on the phase
shifting caused by defects to detect their presence in a phase
shift mask. Phase shift masks change the phase of the light waves
travelling through the mask to expose the photoresist layer on the
semiconductor wafer. The phase shifting (e.g., by 180 degrees)
causes cancellation in particular portions of the light passing
through the mask, thus allowing the printing of smaller features.
FIG. 2A is a cross section of an alternating phase shift mask 200.
The phase shift results from the etched portion 202 of the quartz
mask material 204. Light transmitted through the etched portion
experiences an advanced phase shift relative to light transmitted
through the unetched portions of mask material 204. A typical etch
depth for etch portion 202 is about one wavelength to produce a 180
degree phase shift in a mask with refractive index of 1.5, i.e.
about 250 nm. Example defects can include a bump (e.g., 210 shown
in FIG. 2B), when the etch does not proceed to the full depth in
one or more areas, and a divot (e.g., 211), which may be located on
both the etched surface 212 and unetched surface 214. The detection
techniques of the present invention take advantage of the
diffraction of the incident field when travelling through defects
such as bumps 210 or divots 211.
The invention relies generally on scanner type imaging systems and
detects phase defects on photomasks by their modification of the
light passing through the mask. Much of the detail provided in the
specification for practicing the invention is given for scanners.
Those skilled in the art, having the benefit of the details
provided in this specification, will appreciate that the invention
may also be implemented on projector type imaging systems.
Specifically, the invention relies on the modification of the phase
of the wavefront at the pupil plane of the optical imaging system
using defocus in conjunction with the differential detection of the
image intensity from different segments of a detector in the
detector plane. In a conventional imaging microscope the equivalent
step to having a segmented detector would be to have a segmented
illumination source, which would then require either multiple
passes with different illumination sectors or use of some other
technique, relying on polarization for instance, to be able to
separate the images obtained from different illumination
sectors.
A diagram of an optical scanning microscope system employing
detection methods of the present invention is shown in FIG. 3.
However, the techniques of the present invention may be implemented
with any suitably configured detection system, and the system of
FIG. 3 is not meant to narrow the scope of the invention. These
techniques may be applied to all types of phase shift masks. For
example, the techniques described may be applied to alternating
phase shift masks, attenuated (tritone) masks, and to binary masks.
The incident light inspection beam 302 converges after transmission
through objective lens 304 to a focal point 306. The location of
the focal point may be selected through the use of controller 308.
One skilled in the art would recognize that various means may be
used to control the location of the focal point without departing
from the concepts of the invention disclosed herein. For example, a
stepper motor could be used to move objective lens 304 in order to
locate the focal point 306 at a desired location with respect to
mask 310. Moreover, it should be understood that the principles of
the present invention may be extended to include manual mechanisms
(as well as automatic mechanisms) of selecting the focal point 306.
For example, such manual mechanisms of controlling the location of
the focal point 306 may include rigidly fastening the objective
lens at a desired distance from mask 310 prior to any
inspections.
The inspection light beam 302 is shown transmitted through mask 310
to collector 312. Collector 312 collects the light onto a detector
314 over a range of angles. In conventional systems photo detector
314 sums all intensities of light waves received over the full area
of the detector. The objective lens angle .theta. and the
collection angle .phi. determine in part the signals generated at
the detector. Generally the relationship between the collection
angle .phi. and the objective lens angle .theta. is represented by
.sigma. (sigma), which, for an air medium, is defined as
follows:
Conventional optical inspection systems employ relatively wide
objective lens angles and collection angles, both typically
approximating 45.degree. and resulting in a sigma value
approximating 1. However, large sigma values have been shown to
wash out signals relating to phase defects. The present invention,
in several embodiments, increases the sensitivity to signals
related to phase defects by collecting the light waves onto a
detector 314 split into regions or zones (e.g., radially symmetric
or concentric regions). In practice, it has been observed that
higher intensity signals regarding phase defects are produced when
the sigma is lower. Sigma values in the range of 0.2 to 0.7 produce
suitable results.
Different zones in the detector 314 correspond to different angles
or spatial frequency components of the diffracted light being
collected. What is meant here by the use of the term signal is the
change in detected signal between an object that has a phase defect
and an otherwise identical object without the phase defect, i.e. a
difference signal between patterns. Signals taken from the outside
of the detector experience a reversed sign in comparison to signals
taken from an inside portion of the detector. The present invention
utilizes the reversed sign of the signal to increase the
sensitivity of the apparatus. Experimental results and/or
simulations suggest that a detector wired in a differential mode
produces a larger signal upon detection of phase defects than a
conventionally wired detector. For example, with first and second
detector portions equal in area, the intensity of the resulting
signal taken in differential mode has been found to be nearly 4
times the intensity of a signal taken in a standard or summing mode
from the two detector portions. This results in an increase in the
effect of the defect upon an output image, i.e. this technique is
more sensitive to defects.
Processing of the separate signals generated by the separate
regions or portions of the detector 314 is handled by analyzer 316.
In one embodiment, the analyzer 316 comprises a summing amplifier
which converts one of the detector signals to its negative value
and thus provides a resulting differential signal. In another
embodiment, the separate signals from the detector portions are
summed to produce a resulting signal equivalent to signals obtained
from the full detector using conventional scanning optical
microscopy systems to identify defects, such as pattern defects as
illustrated by defect 108 in FIG. 1B. Analyzer 316 may comprise a
processor with appropriate connective circuitry well known to those
of skill in the art or may include a simple summing circuit without
a connected processor. Alternatively, the analyzer 316 for
analyzing detected signals and the controller 308 for initiating
detection may be integrated in a single device comprising any
suitable combination of software and hardware. It is to be
understood that the invention is not limited in its application to
the details of construction or arrangement of analyzing components
described or illustrated but extends to all configurations wherein
the detector signals may be processed.
FIGS. 4A and 4B depict top views of a first and second embodiment,
respectively, of detector 314. In the first embodiment shown in
FIG. 4A the detector 314' is divided into a first portion 402 and a
second portion 404. In a specific embodiment, first portion 402
comprises a circular region and second portion 404 comprises a
contiguous annular region. The first and second portions are
concentric ideally about the centerline of the light waves
collected by collector 312. In a specific embodiment, first portion
402 and second portion 404 are equal in area. This may be
accomplished, for example, in using the circular and annular
portions described by setting the outer diameter of the circular
first portion 402 to be equal to about 0.7 times the outer diameter
of the annular second portion 404. Each of the portions therefore
represents a portion of the detector corresponding to a smaller
range of angles, i.e., a lower sigma value.
The embodiments of the present invention achieve improved
sensitivity by using a detector split into at least two zones and
by combining those signals from the separate zones in a
differential manner. Suitable results have been obtained when the
detection and the subsequent processing of the signals by the
analyzer have been applied to defocused light beams. While not
wishing to be bound by any theory, it is believed that the
mechanisms involved in producing the larger signal using defocusing
are based on the complex amplitudes of the optical field. Phase
information imaging is described in greater detail in "Fourier
Imaging of Phase Information in Scanning and Conventional Optical
Microscopes", C. J. R. Sheppard and T. Wilson, Philosophical Trans.
of the Royal Society of London, Vol. 295, A1415, February, 1980,
pp. 513-536, which is incorporated fully by reference for all
purposes.
In focussed images direct phase information is lost when the
detector converts the complex amplitude A of the optical field to
an electrical intensity AA. The optical transfer function at focus
is real so for all phase shift objects the image intensity at the
site of the phase defect is nearly the same as it would be in the
case of no phase defect being present. In defocused images the
transfer function has an imaginary part. As a result of this out of
phase, imaginary part of the focused beam, the contrast of phase
defects is greatly increased when defocus is coupled with a low
sigma.
Light scattered from most small phase defects is in quadrature to
the unscattered light. This is undesirable, as a small signal in
quadrature to the unscattered signal will only appear to second
order in the detected signal. That is, if epsilon represent the
strength of the amplitude scattering of the phase defect normalized
to the amplitude of the unscattered plane wave, and the absolute
value of epsilon is small compared to 1, then the image modulation
due to the defect responds in proportion to epsilon.sup.2. Zernike
phase plates and defocus are commonly used to improve phase defect
visibility by getting the defect signal to add linearly, not in
quadrature, to the unscattered signal, so that the defect
modulation is proportional to epsilon, rather than epsilon.sup.2,
and thus produces a higher amplitude signal.
In an alternate embodiment, shown in FIG. 4B, detector 314" is
shown with three zones, a first portion 412, a second portion 414,
and a third portion 416 separating the first portion 412 from the
second portion 414. In this instance, the third portion 416
represents a "dead" band, i.e., an annular isolation region. None
of the light detected by this portion is used to generate an output
signal. Various diameters of the first, second, and third portions
will produce suitable results. For example, a diameter of the first
portion 412 of 0.3 times the outer diameter of the second portion
414, and the outer diameter of the third portion 416 equal to 0.8
times the outer diameter of the second portion 414 works well.
Outer diameters of the first portion from 0.3 to 0.8, and the
second portion having an inner radius from 0.3 to 0.9 work
well.
In other embodiments, the detector may comprise any additional
number of portions, such as, for example, a third portion and a
fourth portion. FIG. 4C depicts a top view of a split detector
314'" having four portions or zones. In one embodiment, the signals
generated from a first portion 421 and a third portion 422 may be
summed and the signals from a second portion 423 and a fourth
portion 424 may also be summed. The resulting summed signals may
then be subtracted to provide a difference signal for analysis. In
other embodiments, two inner zones (i.e., a selective combination
of the detector portions) may be selected from a detector having
three or more portions so as to selectively produce a detector
signal corresponding to an inspection/collection lens system having
a low sigma value. This allows use of a detector which, by virtue
of the multiple detector portions, may present different
sensitivities for different applications by varying the sigma
value. In another embodiment, "active" portions of the detector,
i.e., the portions providing a detector signal for further
analysis, may be separated by isolation regions (426, 427) or
"dead" zones to minimize interference generated in signals in the
active portions of the detector. Though in some cases the isolation
regions may approximate the active regions in size, the invention
is not so limited. The size of the isolation regions may be
minimized or in some embodiments the isolation regions may be
eliminated and still be within the scope of the present invention.
The present invention is not limited to using all of the available
zones in the detector to generate a signal, and, as illustrated
above, is not limited to selecting contiguous zones for generation
of a summed signal. Specific zones or portions may be "dead" zones
and the zones selected for summing may be contiguous or not,
selected in accordance with the sensitivity desired.
Another advantage to having multiple annular detector segments is
that it allows for flexible operation with different pixels sizes.
For instance, larger pixels are used at times for mask inspection.
This is accomplished by reducing the NA of the objective lens.
Since the sigma of the imaging system is given by the ratio of the
sine of the collection angle to the sine of NA, changing NA for
different pixel sizes requires changing the sine of the collection
angle if sigma is to remain the same. If sigma were allowed to
increase, as it would if the NA were reduced and no change were
made in the detector configuration, the microscope's phase
sensitivity would decrease. One way to do this is to have a
detector with multiple annular regions so that when the NA is
reduced elements closer to the center can be used, thus keeping
sigma constant with varying pixel size.
In another embodiment, a Zernike phase shift plate or similar phase
plate is introduced in the pupil plane to improve phase sensitivity
in lieu of defocusing. Zernike techniques provide a mechanism for
emphasizing small phase steps.
At the left of FIG. 5 an object 502 having a low contrast phase
defect 503 is illuminated, and at the right a phase contrast image
504 of this object is produced. The phase contrast image 504 is
produced by forming an intermediate Fourier plane 506 of the image
and inserting a Zernike phase contrast filter 508 in this plane.
This filter 508 shifts the amplitudes of the low spatial frequency
components of the image 504, which forms the background of the
image 504, by 90 degrees. It may also attenuate the low spatial
frequency components of the image 504 as well, which has the effect
of reducing the overall brightness of the image but increasing the
contrast of phase defects in the image, which is sometimes
desirable. The phase contrast is increased by this filter 508
because when the output image is formed from this filtered Fourier
plane 506 the background is now collinear in amplitude with the
components of the image due to the phase defect. Therefore the
signals add linearly rather than in quadrature. For instance, a
small phase defect that is 0.1 of the amplitude of the background
that is imaged using a phase contrast technique would give an
approximately 10% modulation of the image at the point of the
defect. However, if this amplitude were in quadrature rather than
in phase it would only give approximately a 1% modulation of the
image, which would make it much harder to detect.
A defocused imaging system is somewhat like a focused system with a
(Zernike) phase plate where the phase shift increases quadratically
with spatial frequency (or pupil radius). A Zernike plate or a
complex amplitude plate may be used to produce a phase shift
between the regions of the detector, such as between the annular
and central regions. Zernike plates usually specify a 90 degree
phase shift and usually are attenuating for the direct light (or
zero order). Those skilled in the art with the benefit of this
specification would recognize that complex amplitude plates may be
selected to produce variations in both phase and amplitude to meet
a desired sensitivity.
Typically, the signals generated from the analyzer are connected to
a display generator 318 (see FIG. 3) in order to create a display
of the inspected mask. The display generator may comprise a CRT, an
LCD screen, or other suitable display device coupled with
appropriate processing circuitry to convert the resulting voltage
signals received as the light beam scans the mask to an image on
the display screen of the display generator 318. Generally, a phase
defect image derived from an inspected mask will show edges of
layers in addition to phase defects. For example, a difference
image may be obtained from two identical areas of the mask to
isolate the images generated by the phase defects. If the
difference image contains pixel values above a certain threshold,
then a defect has been identified. This threshold is set so that
defects above a certain specified size are found reliably, while
the number of false and/or nuisance defects is kept acceptably
low.
In one embodiment, the techniques of the present invention permit
pattern defect inspections and phase shift inspections to proceed
using a single scanning optical microscopy system in two passes.
The number of passes depends on the algorithms, noise, and desired
defect sensitivities.
FIG. 6 is a flowchart illustrating an inspection technique for
phase shift defects and for pattern defects in accordance with one
embodiment of the present invention. In general terms, the system
completes pattern and phase defect inspections in two passes over
the photomask to be inspected. The first pass commences with an
inspection for phase defects in the photomask (604). Phase defect
inspection, in accordance with several embodiments, requires a
defocused image. The focus is set to a predetermined distance in
one direction from the surface of the photomask (606). Suitable
distances will vary as a function of the wavelength (lambda) of the
impinging light waves and numerical aperture (NA) of the objective
lens, specifically in proportion to lambda/NA.sup.2. For example, a
suitable range lies from a distance 0.5 to 3*lambda/NA.sup.2.
Suitable results have been obtained in the range from about 200 nm
to about 500 nm but these values may vary according to the factors
as discussed above. The scan commences with the incident light
directed to a spot on the photomask and the collection of the
resulting signals from a first and second portion of the detector
(608). In this first mode, a differential signal is generated from
separate portions of the detector (610). Subsequently, the
resulting signal is used to generate an image of the mask to
identify defects (612). A bright or dark picture element will be
generated to correspond to the scanned spot. If the scan of the
photomask is not complete (614), the scanning continues over the
next section of the photomask (616). Various mechanisms may be used
to perform the scanning as is known to those of skill in the art.
For example, in one embodiment, the light is focused on a spot
followed by the generation of detector signals. In order to perform
the two-dimensional scan, the photomask may be moved in one
direction and the beam moved in a direction perpendicular to the
mask movement.
Once the scanning of the photomask for phase defects is complete,
the second pass commences (618). Initially, in this conventional
inspection mode, the focus is set to the surface of the photomask
(620). This location is in fact typically set to the surface of the
chrome layers patterned on top of the quartz layer of the
photomask. In this mode, the signals are collected from the full
detector by generating the sum of signals from the separate
portions of the detector (622 and 624). As with the case of the
phase defect inspection, the resulting signal is used to generate a
picture element of an image displaying pattern defects (626).
Scanning of the mask continues for pattern defects if the image is
not complete (628, 630). Although this embodiment is shown
commencing with an inspection for phase defects in a first mode
followed by an inspection for pattern defects in a second mode, it
will be recognized by those skilled in the art that the order of
inspections may easily be interchanged without departing from the
basic principles of the invention.
Although the foregoing invention has been described in some detail
for purposes of clarity of understanding, it will be apparent that
certain changes and modifications may be practiced within the scope
of the appended claims. It should be noted that there are many
alternative ways of implementing both the method and apparatus of
the present invention. Accordingly, the present embodiments are to
be considered as illustrative and not restrictive, and the
invention is not to be limited to the details given herein, but may
be modified within the scope and equivalents of the appended
claims.
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